The invention pertains to the field of YIG tuned frequency synthesizers. More particularly, the invention pertains to the field of switched YIG direct synthesizers for generating low noise, nanosecond tuned microwave signals.
In the field of microwave components, small size, low power dissipation, very fast tuning speeds, low phase noise and low unit cost are very important characteristics. Frequency synthesizers in the microwave components area are devices that generate signals at tunable frequencies with a high degree of accuracy. It is highly desirable to be able to rapidly tune the frequency of the frequency synthesizer and to continually sweep the frequency of the output signal throughout the range of frequencies that can be generated. Generally, the output frequency of the frequency synthesizer is used as the local oscillator signal in a superhetrodyne receiver or other apparatus where a local oscillator or variable frequency source is needed.
An embodiment of a prior art synthesizer, which is believed to be the closest known prior art to the invention claimed herein, was marketed by ESSI of Fremont, Calif. in 1990 and earlier under the model number ER 3300 and ER 3400. In this embodiment, a 500 MHz surface acoustic wave oscillator, fed a power amplifier which fed a step recovery diode comb line generator. A reverse slope equalizer equalized the power in the comb lines and had its output coupled to a power divider which divided the power between two tunable FERRETRAC.TM. YIG passband filters of the type described in U.S. Pat. No. 4,127,819, the contents of which are hereby incorporated by reference. These tunable filters were used in ping-pong fashion to select alternate comb lines for coupling to a mixer. The output of the filters were coupled through switchable power amplifiers to implement an output switch to a power divider to match the amplifiers and thence to a mixer. The mixer also received a local oscillator signal from a voltage controlled oscillator having an output frequency range from 250-500 MHz. The mixer up converted the selected comb line and output two sideband frequencies and the "carrier" to a power amplifier. These signals were coupled to the input of another FERRETRAC YIG passband filter which selected the desired sideband signal. The output of the filter was then amplified and made available for use as a synthesized signal.
In another embodiment in the prior art, a YIG frequency synthesizer is comprised of a YIG oscillator in a phase locked loop with a feedback path back to the oscillator from the output. A sample of the output frequency of the YIG oscillator is mixed with a selected comb line frequency to generate an intermediate frequency that is compared with an intermediate frequency in a phase detector to generate an error signal which is fed back to control the frequency of the YIG oscillator. The comb line frequency was generated by a YIG filter coupled to the output of a step recovery diode driven by a source,oscillator. The step recovery diode generates a spectrum of harmonics, one of which was selected by a tunable YIG passband filter for application to the mixer. Fine tuning of the output frequency is achieved by changing the frequency of the reference oscillator.
This type of frequency synthesizer, manufactured by Hewlett Packard and others, is very slow, having a tuning speed of around 10-50 milliseconds. The basic problem with this architecture is that it cannot be tuned continuously throughout the range nor can it be tuned rapidly. The bandwidth of the feedback loop is narrow which prevents rapid changes of the YIG oscillator frequency. To get around this problem, this type filter is normally tuned with the feedback loop open to get the YIG oscillator approximately at the desired frequency, and then the feedback loop is closed. Upon closure, the YIG oscillator takes 10-50 milliseconds to stabilize at some frequency which may be the incorrect frequency and which may need further adjustment. Further millisecond delays may be imposed by the need to alter the center frequency of the YIG passband filter to select a new comb line. Because of the need to open the feedback path, the output frequency cannot be continuously altered throughout the range of possible output frequencies.
Another type of frequency synthesizer exists in the prior art which can tune faster than the preceding examples of prior art. These types of frequency synthesizers, typified by the devices manufactured by Comstron of Long Island, N.Y., use a multiply and divide architecture to manipulate a base frequency up or down to the desired frequency. Comstron units are available which can switch frequencies in from 1 microsecond to 100 nanoseconds. Unfortunately this type of frequency synthesizer is very large and heavy and can weigh as much as 50 pounds. Also, these type units are very expensive.
Conventional broadband microwave synthesizers are rack mounted instruments weighing 50 pounds or more and consume hundreds of watts of power.
Therefore, a need has arisen for a new type of small, low cost, low power consumption frequency synthesizer which can change frequencies very fast, i.e., on the order of one microsecond or faster with a very broad range of output frequencies, and having high selectivity, high rejection of unwanted signals and excellent frequency resolution.